How to Work with Celastic
Celastic (generic name) is a plastic impregnated fabric which becomes moldable and adhesive when activated by immersion in solvent or with heat. When dry or cool, it is transformed into a light-weight, high impact, weatherproof "shell", having excellent shape memory and bonding power. Celastic can be drilled and sanded and will accept many finishing techniques. The brand of celastic which we use and sell is "FORM-FAST®".
FORM-FAST is a smoothly textured sheet, white to off-white in color, enabling the most delicate contours and finishes.
Celastic can be used with various types of molds: positive, negative or free-form draping. We use both positive and negative molds in our work. The positive molds that we use for character heads are made of concrete since the finished celastic must be cut so that it can be removed from the mold. The negative molds that we use are made of fiberglass which are light-weight and easy to store. We use these materials because our mold must stand up to repeated use. However, for a short-time use, the molds can be made of almost anything. Each type of mold has it's pros and cons.
For the purposes of this tutorial. we will be concentrating on using celastic with a positive mold. The technique for using celastic with a negative mold is essentially the same as the positive molding technique. The exceptions will be noted as we go along.
Celastic is activated with the use of either Acetone, Methyl Ethyl Ketone or a combination of both. Acetone has a shorter curing time, but has less tack during it's usable period. MEK has a longer curing time and more tack during the working period. To take advantage of the properties of both activators, we use a mixture of 3 Parts Acetone to 1 Part MEK.
As with many chemicals, both Acetone and MEK need to be handled with care and the use common sense. Use the following guidelines when working with celastic:
For the purposes of this tutorial, we will be making a character head. However, the principles explained are applicible to almost any project using celastic.
When celastic is softened in the activator, it becomes tacky so that it will stick to itself and form a solid product when dry. It will also stick to the molds. Therefore, you must prepare the molds with a parting agent before applying the celastic. Parting agents can be wet newspaper, aluminum foil, plastic wrap or a "PAV" parting film.
For our positive molds we use aluminum foil. It conforms easily to the shape, pulls away from the mold when the celastic shell is removed and easily peels away from the interior of the celastic shell. For negative molds we use a "PAV" parting film which is applied to the molds surface with a brush an allowed to dry. When the celastic shell is removed from the mold, the film peels away from the celastic and the mold can be cleaned with water.
The photo to the right is one of our molds covered with aluminum foil. The mold has a lot of detail which must be transferred to the finished celastic shell.
For a negative mold you will want to use the PAV parting agent only.
Preparing & Using the Celastic
Celastic is applied to the mold in the same way that you would apply papier maché, ie., workable strips are softened in the activator and applied to the mold one at a time. One point to remember is that the more detailed the surface, the smaller the pieces of celastic need to be. In the photo to the right, you can see that some of the surfaces are smooth and relatively flat, while the facial features are rather detailed. Larger pieces of celastic would be used to cover the hat, ears and back of the head. Smaller pieces would be used for covering the face.
Remember that celastic is a woven fabric that is coated and impregnated with a plastic. It will behave like a piece of fabric. It has a length-wise and cross-wise grain and it will stretch on the bias (diagonal grain) when softened. The edges along the length of the fabric are the selvedges.
Begin preparing the celastic by tearing strips 2" to 3" wide from selvedge to selvedge. Then tear these strips into 4" to 6" pieces. Always tear the fabric as opposed to cutting with scissors. The torn pieces provide ragged edges for feathering the pieces into one another so the laps are not so obvious and harsh. This is especially important if you plan to paint the finished piece.
Once you have your pieces torn, your are ready to begin applying to the mold. Immerse 6 to 8 pieces of celastic in the Acetone/MEK mixture. It will take from 10 to 30 seconds for the pieces to become workable. Remove all of the pieces from the mixture and drape them over the side of the container until you need them. This prevents the plastic from completely dissolving out of the fabric. The longer the celastic is left in the solution, the more plastic is lost. If the pieces begin to stiffen before you get to them, you can always dip them in the solution again. The celastic is best used when it reaches the feel and drape of garment leather.
Lay one piece at a time onto the mold, smoothing them from the center of the piece to the edges. Make sure that no air bubbles form between the celastic and the mold or between layers of celastic. Lay a second piece of celastic next to the first, lapping the edges about 1/4". This provides a good seal between pieces and makes for a strong shell. Continue laying piece after piece in an orderly manner until the entire mold is covered. Try to lay all of the pieces in the same direction. Neatness counts! When you begin to lay the second layer, you will lay the pieces in the opposite direction. By doing this, you will be able to keep track of the areas you have covered a second time. This helps prevent ending up with very thick or very thin places in the finished product.
Once the first layer has been completed, allow it to cure for 3 to four hours before applying the second layer. You do this because as the plastic coating solidifies on the outside it forms an impervious barrier which retards the drying of the interior of the celastic shell. By allowing the first layer to cure slightly, the interior of the shell has a chance to begin to cure before the second layer makes it even more air tight. The first layer does not have to be completely dry before you begin the second layer.
If you are using a negative mold, follow the above procedure for using celastic. The one difference is that you want to keep the celastic about 1/4" to 1/2" away from the edges of the mold that will be joined together in the next step.
Parts of a multiple-piece
Unmolding the Shell
After you have finished laying-up the celastic shell it needs to dry or cure for 12 to 24 hours, depending on the relative temperature and humidity in your work area. The drying time can be speeded up with the use of fans. But remember, the fumes form the solvents are heavier than the air and thus stay near the floor. Fans should be placed on the work table or on floor stands to lessen the possibility of sparking the fumes.
To remove the shell from a positive mold, you need to cut the shell so that it can be pulled away from the mold. Usually the sides are the best place to make the cuts. Leave a 3" to 4" space on top of the shell uncut to act as hinge. This makes re-assembling the two halves a little easier. Until you are comfortable with cutting the shell off of the mold, it is a good idea to draw you cutting lines on the shell. Also you will want to strike balance marks across the cutting line which will be lined up when you re-assemble the shell.
The shell may be a little difficult to remove due to the fact that the celastic shrinks as it dries, making it conform very tightly to the mold. Work the celastic loose little by little, starting at the bottom of the mold and working up the sides. Work carefully since there is a possibility of the celastic tearing. It just takes patience and a lot of elbow grease.
Once the shell has been removed from the mold, you need to strip the release agent from the interior. Most of the foil or paper or whatever will probably stick to the shell. As you peel the foil away, you will notice that the inside of the shell is still damp. Do not let the inside dry before you re-assemble the two halves of the shell. If you do the shell will warp and be very difficult to put together.
To assemble the shell use strips of celastic between 1" and 2" wide and about 6" long. Hot glue these strips lengthwise along one cut edge on the interior of the shell. Butt the corresponding cut edge to its mate and hot glue the in place. Repeat for the other side. These strips that are used to dutchman the halves together are not softened in the activator before they are hot-glued in place. If the cut edges do not meet completely, you can use the hot glue as a caulk to fill in the gaps.
Once the cut edges have been glued into place, the seam is covered inside and out with strips of softened celastic. The softened celastic that covers the interior gluing strips will bond with the unsoftened celastic and form a very durable seam. Allow the newly applied strips and the interior of the shell to dry completely.
Finishing the Shell
Once the shell has been reassembled and the stripping pieces have dried there are a few things that need to be done to finish the shell.
As you may notice in the photo of the previous section, the neckline is rather ragged. These ragged edges need to be trimmed off to even up the neckline. After this edge is cut, it is a good idea to bind the cut edge with additional pieces of celastic. This both neatens and reinforces the cut neck edge. All edges that are likely to take any type of stress of abuse need to be bound with extra pieces of celastic.
Another step that needs attention at this point is to plan and cut all openings for vision and ventillation. It is not critical that this be done at this stage in the process. But if you are going to make patterns for the covering of the head, it is helpful to have these areas cut out.
The cutting and trimming steps above can be accomplished with several types of tools. Our tool of choice is a Dremel® rotary tool. We use the fiberglass reinforced cutting disk to remove the shell from the mold and to trim the neckline. To cut out the vision and ventillation openings we use various sizes of the spiral cutters. Remember: when using any type of power tool, be sure to read and understand all operating and safety instructions.
And the last thing that you want to do at this point, is to paint the interion of the shell black. This helpsabsorb any light that enters the head through the vision and ventillation openings and makes the wearer's head less visible by the audience.
There are many ways to finish your celastic project, depending upon the the effect you are trying to achive. Our method of choice is to cover our character heads with acrylic faux fur, polar fleece or some other type of
We always begin this process by making a pattern for the covering. This involves smoothing and pinning muslin over the surface of the shell to establish the exact shapes the covering fabric needs to be. Pattern 1/2 the front of the head from the center front to the side. Then pattern 1/2 of the back from the side to the center back. Transfer the muslin patterns to paper and you are ready to cut the covering. Be sure to cut a right and left front and a right and left back. The more complicated the head, the more pattern pieces you will need for the cover. For example, our project head uses three pattern pieces for the entire fur cover: one complete front pattern, one complete back pattern and one ear pattern. Please note that this description of the draping process is a very simplistic explanation. For a very detailed surface it can becom a very complicated process. Perhaps a topic for a future turtorial.
Another way of finishing your project is to paint the surface. This can entail the following steps, depending upon the final look you want.
Some Final Thoughts
Celastic has been used for constructing theatrical scenery, props and costumes for at least 30 years. I first became acquainted with the product while in college during the late 1960's. It's virtues are that it is light-weight, extremely durable, waterproof, and easily tooled and finished. It's main drawback is that the activating chemicals are rather noxious and can be a health hazard if mis-handled. Let your common sense rule when using these or any chemicals.
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©Schenz Theatrical Supply 1999